The present invention relates to an apparatus and method for determining the density of test materials, and more particularly to a nuclear radiation measurement apparatus and method for measuring the density of soil, asphaltic materials and the like during movement of the radiation measurement apparatus across the surface of the material which is undergoing measurement.
Nuclear radiation gauges for determining the density of soil and asphaltic materials are well-known, as described for example in U.S. Pat. No. 2,781,453. Such gauges employ the phenomenon of scattering of gamma rays and are known by those skilled in the art as "scatter" gauges.
Such gauges typically take the form of a hand held portable instrument which is positioned on the surface of the test material for a predetermined period of time while backscattered radiation is counted to obtain a density reading. Devices of this type have been widely used and well accepted in the industry for obtaining rapid non-destructive density measurements of the test material. The density gauges are particularly useful in determining the degree of compaction of soil or asphalt during the construction of roadbeds and pavement surfaces, in which heavy rollers or compactors are rolled back and forth across the surface and density readings are made periodically using the portable stationary nuclear density gauges of the type described above.
It has been recognized that it would be quite desirable to obtain a readout of density continuously during the compaction operation, rather than periodic spot density readings. This approach would give a density reading over a large area rather than an instantaneous spot reading, and would also make it possible to more rapidly respond to changes in the density readouts during the compaction operation. To this end, several movable, dynamically reading nuclear density gauges have been proposed. One such gauge is in the form of a wheeled unit and employs a pair of small cylindrical rolls as wheels with a nuclear source and detector mounted between the rolls in a suspended, noncontacting relationship with the underlying test surface. The gauge rides along the pavement surface and may be connected to and pulled by a pavement compactor vehicle. Another such gauge, described in published European Patent Application No. 108,845, has the nuclear radiation source and detector mounted inside a cylindrical roll, and the roll may be manually pushed along the pavement surface or propelled therealong by connecting it to a pavement compactor vehicle.
Both of these gauges assume a constant spacing (air gap) between the source/detector system and the underlying pavement surface. However, the density reading obtained from a backscatter gauge through an air gap is quite sensitive to variations in the size of the gap. During operation, any buildup of asphalt on the rolls will increase the effective diameter of the rolls and alter the size of the air gap between the source/detector system and the surface of the underlying test material, introducing error in the density reading.
Mobile nuclear density gauges are also disclosed in U.S. Pat. Nos. 3,341,706 and 3,354,310. These patents disclose a mobile nuclear gauge which can be towed by a vehicle along a roadway or surface to obtain a continuous logging of the density and moisture. The gauge is mounted in a trailer above the road surface. The patents recognize that in order to obtain an accurate measurement, the air gap between the gauge and the surface must be maintained as nearly constant as possible, and they attempt to accomplish this by providing soft tires on the trailer vehicle so that roughness is absorbed by the soft tires. The '706 patent mentions that as an alternative to maintaining a constant air gap, it may be desirable to monitor the air gap by suitable optic, mechanical, nuclear or sonic means and to compensate for the differences which may occur in the air gap. However, the patent gives no indication of how this compensation might be accomplished.
It is an object of the present invention to provide a nuclear density measurement method and apparatus for dynamically measuring pavement density and which overcomes the above-noted disadvantages and limitations of the prior art.
It is a further object of the present invention to provide a nuclear density measurement method and apparatus which is particularly suited to be used by a pavement compactor vehicle during a pavement compaction operation.